This invention relates to a current drive circuit for a servovalve and more particularly to a current drive circuit for a servovalve which produces a fixed frequency bipolar rectangular waveform having a duty cycle dependent upon the magnitude of an input digital word. Servovalves of the electrohydraulic type have been used broadly as the interface between an electrical control signal and different types of actuating and metering devices. One such application is in fuel control systems for gas turbine engines. Typically in such fuel control systems there is an electrical signal generated by the control which corresponds to the difference between a reference engine speed and the operating engine speed. This electrical signal is connected to the input of a servovalve which controls a servopiston which in turn is mechanically coupled to a fuel metering valve. In this manner, fuel flow of the gas turbine engine is varied as a function of the electrical signal in order to maintain the reference engine speed. Such a system provides a highly stable and accurate control of engine speed. Today's applications for servovalves, particularly in gas turbine engines, demand that the servovalve be fail-fixed. By fail-fixed, it is meant that the mechanical output of the servopiston, as may be provided by an actuator, will be locked in place immediately following a loss of either the electrical input signal or the hydraulic lines.
One such fail-fixed servovalve is disclosed in U.S. Pat. No. 3,922,955 issued to Howard B. Kast on Dec. 2, 1975, and assigned to the assignee of this invention. The servovalve disclosed in the Kast patent includes a flexible jet pipe for ejecting a stream of pressurized liquid into a receiver passage disposed within a chamber. The direction of deflection of the jet pipe is controlled by a torque motor having an armature secured to the jet pipe so as to exert a bending moment thereon. The servomotor in turn is activated by current drive signals. As more fully described in the above-referenced Kast patent, the directon and amount of deflection of the jet pipe is dependent upon the polarity and magnitude respectively of the input current drive signal and the servopiston is disposed so as to remain in the position it was in at the time of failure in the event of a complete loss of the driving current or of a hard over failure of the driving current.
The present invention is directed to an improved driving circuit for a fail-fixed servovalve of the type disclosed in the above-referenced Kast patent which is adapted to interface with digital control circuitry. Prior art circuits of this type have generally included several analog components, (e.g., sample and hold circuits) and have exhibited undesired inaccuracies due to drift of such analog components.